The standard technique for enhancing strength of polypropylene fiber comprises increasing the draft ratio. In addition, various studies on stretching conditions, spinning conditions, molecular weight of polypropylene, and the like have been conducted for the same purpose.
For example, JP-B-54-32855 discloses a method comprising decreasing the molecular weight of polypropylene within a pelletizer. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
Other methods for enhancing strength include a method in which spinning is carried out by using a polypropylene resin with high crystallinity is known. There have been made some proposals to attempt to improve the crystallinity of polypropylene. For example, it that when aluminum salts or sodium salts of aromatic carboxylic acids (see JP-A-58-80329), or aromatic carboxylic acids, aromatic phosphoric acid metal salts or sorbitol derivatives (see JP-B-55-12460, JP-A-58-129036, and U.S. Pat. No. 3,207,739) are added, they function as a nucleus-generating agent (hereinafter referred to as a "nucleating agent") for crystal nuclei to thereby improve the crystallinity. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, among these nucleating agents, although the sorbitol derivatives exhibit an excellent nucleating effect, they usually bleed out of the resin and cause roll staining at spinning or generate a strong odor at processing; therefore, they are limited in terms of utility.
Moreover though the aluminum salts of aromatic carboxylic acids which have generally been used function as a nucleating agent, they generate a number of voids at molding of polypropylene or cause stretch cutting at stretching and, hence, have not always been satisfactory.
U.S. Pat. No. 4,551,501 discloses a process for adding a vinylcycloalkane polymer in order to improve the crystallinity of polypropylene and teaches that such a crystalline propylene polymer can be applied to fibers. However, the teachings of U.S. Pat. No. 4,551,501 are very general, and this U.S. patent does not suggest any requirement for obtaining high-strength polypropylene fiber.
Besides, U.S. Pat. No. 3,887,534 discloses a method in which a crystalline propylene polymer is degraded with a peroxide to improve processability. However, this U.S. patent does not at all suggest that in the case of production of polypropylene fiber, stretchability is improved to obtain a high-stength fiber.
As described above, any of the conventional techniques has a limit in achievement of strength of polypropylene fiber strength. It has been demanded that polypropylene fiber strength be further improved.